1. Field of the Invention
The present invention is in the field of medical articles and more specifically is an improvement to a hypodermic syringe for the purpose of making the syringe more tamper-resistant.
2. The Prior Art
The present invention is an improvement to a hypodermic syringe of the type described in U.S. Pat. No. 3,941,129, issued Mar. 2, 1976 to Ida M. Pleznac, the present inventor. The invention of that patent is illustrated in FIG. 1 and FIG. 2 herein. These figures show the end of a hypodermic syringe that is opposite the end from which the needle extends.
In common with other hypodermic syringes, all of the syringes shown herein in FIGS. 1-5 include a tubular member 12, typically made of a transparent material, such as glass, and having a constant cross-section. The tubular member 12 contains the fluid 14 to be injected. The fluid is expressed from the syringe by applying a force to the drive piston 16, the force being directed to the left in FIGS. 1-5, and being applied by an actuator rod (not shown) which the user screws onto the screw 18 that is embedded in the drive piston 16. The drive piston 16 is made of a resilient material, such as a soft rubber, so that it will maintain a sealing engagement with the inside wall of the tubular member 12 as the drive piston 16 is forced to the left as viewed in FIGS. 1-5.
The invention described in the aforementioned U.S. Pat. No. 3,941,129 differs from previous hypodermic syringes in having a non-retractable stopper 20 within the tubular member 12 and to the left of the drive piston 16. The non-retractable stopper 20 includes a longitudinal passage 22 for the fluid 14 to flow through. The non-retractable stopper is not connected to the drive piston 16, although they may be in contact sometimes.
FIG. 1 shows the syringe as it would be initially supplied from the manufacturer, but prior to use. Note that the non-retractable stopper 20 is in contact with the drive piston 16. In use, the drive piston 16 is pushed to the left as viewed in the figures, driving before it the non-retractable stopper 20, as fluid is expressed from the syringe. The drive piston 16 may be left at the position to which it had been advanced leftward, or alternatively, the drive piston 16 may be retracted to the right, particularly if the fluid originally expressed is to be replaced by a substitute fluid. Friction holds the non-retractable stopper 20 at the position of its greatest leftward advance within the tubular member 12 even when the drive piston 16 is retracted, as shown in FIG. 2. The space between the drive piston 16 and the non-retractable stopper 20 is filled by a flow of fluid 14 through the longitudinal passage 22. All of the syringes shown in FIGS. 1-5 herein work in this same way.
Clearly, the use of the non-retractable stopper 20 is an improvement over the previous art because it enables a user to determine whether a substantial amount of fluid has been removed or has been removed and replaced. However, if only a rather small amount of fluid has been removed, discovery of the prior use of the syringe is less likely. The present invention is a simple modification of the improved syringe shown in FIGS. 1 and 2. This modification has as its purpose to facilitate detection of the removal of even relatively small amounts of fluid from the syringe.